Waves do not bend when they enter a new medium if they are incident at a zero angle. This means the waves are traveling perpendicular to the boundary between the two mediums.

Here's why:

* Refraction is the bending of waves as they pass from one medium to another. This bending occurs because the speed of the waves changes in the new medium.

* Angle of incidence is the angle between the incoming wave and the normal (a line perpendicular to the boundary).

* When the angle of incidence is zero, the wave is traveling directly into the new medium, there's no change in direction, and therefore no bending.

Example: Imagine a light ray shining directly down onto a flat surface of water. The light ray will enter the water without changing direction because it hits the surface at a zero angle.